Optical disk apparatus and tracking method

ABSTRACT

A tracking offset is reduced. The effect of an SPP signal fluctuation is reduced by using a variable mixing ratio DPP method, and a residual tracking offset is minimized.

CLAIM OF PRIORITY

The present application claims priority from Japanese application JP2006-169016 filed on Jun. 19, 2006, the content of which is herebyincorporated by reference into this application.

FIELD OF THE INVENTION

The present invention relates to an optical disk apparatus, and moreparticularly, to a tracking signal detection system which enables atracking control offset to be reduced when a differential push-pull(DPP) method is used.

BACKGROUND OF THE INVENTION

The DPP method in which the offset of a tracking control signalresulting from lens shifting can be effectively suppressed has been inwide use as a tracking system for optical disks. In the method disclosedin JP-A 54415/1993, three spots for tracking, i.e. a main spot, a firstsub-spot, and a second sub-spot are positioned being shifted by half atrack pitch each on a land/groove disk or a groove recording disk, andthe offset of the tracking control signal caused by lens shifting is, inprinciple, canceled based on calculations made using three push-pullsignals detected. When the DPP method is used in an isolated opticalsystem, there are cases in which optical axis deviation causes themagnitudes of the three push-pull signals to vary. A method aimed atsolving the problem is disclosed in JP-A No. 89494/1993. In the method,gain adjustment is made, in a mirror section, between the push-pullsignal of a main spot and the push-pull signals of sub-spots. In themethod disclosed in JP-A No. 279659/2002, to make up for a difference inreflectivity between land and groove, distinction is made between landand groove and between media, and the push-pull signal of eithersub-spot is selected for use. In the method disclosed in JP-A No.320287/1995, a function for selecting the three-spot method or the DPPmethod according to the medium type is used.

SUMMARY OF THE INVENTION

The emergence of dual-layer media has brought about a problem additionalto those mentioned above. Namely, a tracking offset is caused by lightwhich is reflected from other than a recording layer and leaks into therecording layer. Besides, many media manufacturers have started sellingmedia varying in groove depth and other optical conditions, so that ithas become necessary for optical disk drives to incorporate trackingcontrol measures compatible with different media.

In connection with the problem of light leaking into a recording layerfrom a different layer when tracking control is performed on amultilayer medium by the DPP method, it is stated in JP-A No.161295/1997 that the stray light from a different layer is DC light withno signal component. But, considering that the light leaking into arecording layer from a different layer is caused by interference betweenthe recording layer and the different layer, such light cannot beuniformly removed.

FIGS. 2A and 2B are diagrams showing results of measuring push-pullsignals during reproduction from a dual-layer disk. FIG. 2A is a diagramshowing measurements of the push-pull signal of a main spot (hereinafterreferred to as the “MPP” signal). FIG. 2B is a diagram showingmeasurements of the push-pull signal of a sub-spot (hereinafter referredto as the “SPP” signal). In a general DPP method, the MPP signal and theSPP signal are adjusted to be of a same polarity and to have a sameamplitude, and the DPP signal is defined by the following equation.DPP=MPP−SPP  (1)

Referring to FIGS. 2A and 2B, the MPP and SPP signals are adjusted tohave an amplitude in a range of ±20 mV, but, whereas the amplitude ofthe MPP signal is stable mostly remaining within the range, theamplitude of the SPP signal shows large fluctuations. The fluctuationsare reflected on the DPP signal according to the equation (1). Suchfluctuations of the SPP signal are considered attributable to variationsin interlayer spacing and interference between grooves of the twolayers.

FIG. 3 is a schematic diagram showing the theory of interlayer crosstalkgeneration which is a factor in the SPP signal fluctuation on adual-layer disk. An optical spot is focused on the first layer, and thelight reflected from the first layer enters three light receiving areasof a photo-detector. At this time, the light not focused on the firstlayer is reflected from the second layer, so that the photo-detectordetects the light leaking in from the second layer, too. Generally, thelight quantity of a main spot is several times larger than the lightquantity of a sub-spot, so that the main spot on a defocused layerinterferes with the sub-spots on the recording layer causing the SPPsignals to fluctuate.

In the case of single-layer disks, too, tracking by the DPP method in anoptimum power control (OPC) area becomes unstable. FIG. 4 shows asimulated DPP signal in an OPC area. Compared with the DPP signalamplitude in an unrecorded area shown on the left in FIG. 4, theamplitude in a recorded area shown on the right is small. This isbecause the record marks formed in the recorded area lower thereflectivity of the area. Singular points, each marked with a circle inFIG. 4, appear in a border area between the recorded and unrecordedareas. This is attributable to imbalances between the MPP and SPPsignals caused when the main spot and sub-spots scan areas withdifferent reflectivities. Such singular points disable servo control,thereby causing a tracking error.

FIG. 5 gives an enumeration of conceivable causes for a tracking offset.The conceivable causes are as follows. Indications “(MPP)” and “(SPP)”denote whether an offset is caused to the main spot push-pull signal(MPP) or to the sub-spot push-pull signal (SPP).

1. Causes originating in optical system

(1) Sub-beam displacement due to diffraction grating misalignment (SPP)

(2) Collimation and beam shaping errors (MPP), (SPP)

(3) Detector misalignment (MPP), (SPP)

(4) Misalignment of laser wavelength and optical component wavelengthsresulting from temperature changes (MPP), (SPP)

(5) Misalignment of laser wavelength and optical component wavelengthsresulting from aging (MPP), (SPP)

2. Causes originating in media

(1) Optical axis displacement due to tangential tilt (MPP), (SPP)

(2) Optical axis displacement due to radial tilt (MPP), (SPP)

(3) Interlayer interference in dual-layer disk (SPP)

(4) Change in reflected light quantity after recording (MPP), (SPP)

(5) Intergroove interference in dual-layer disk (SPP)

3. Causes originating in servo system

(1) Tracking error (MPP), (SPP)

The present invention is aimed at reducing an offset attributable to SPPsignal fluctuation or an unbalanced state of the signal and stabilizingtracking control.

An offset caused by SPP signal fluctuation can be reduced by reducingthe involvement of the SPP signal in the DPP signal. Lowering the ratioof the SPP signal, however, decreases the effect of offset suppressionby lens shifting. A solution is to use a normal DPP signal when the SPPsignal does not fluctuate and reduce the SPP signal when shifting of thelens is adequately small. Namely, the mixing ratio between the MPP andSPP signals is to be changed according to the condition. This methodwill, hereinafter, be referred to as the variable mixing ratio DPP(VDPP) method. The VDPP signal is defined by the following equation.VDPP=αMPP−(1−α)SPP  (2)

where the coefficient α(0≦α≦1) represents the MPP mixing ratio. Whenα=0.5, VDPP is equivalent to DPP.

Setting α to be smaller than 0.5 to reduce the effect of SPP signalfluctuation decreases the effect of lens shifting to suppress a trackingoffset. It is therefore necessary to learn a value of the mixing ratio αwhich minimizes the residual tracking offset. To determine anappropriate value of the mixing ratio α to be applied when using thevariable mixing ratio DPP method, it is necessary to quantify (1) SPPsignal fluctuation resulting from interlayer light leakage in adual-layer disk, (2) tracking offset caused by lens shifting, and (3)residual tracking offset.

With reference to FIG. 6A showing MPP and SPP signals having undergoneoffset and gain adjustment, how to measure SPP signal fluctuation willbe described. In this case, as the MPP signal is ideally the same as theSPP signal, it may be considered that the amount of SPP signalfluctuation equals the difference between the MPP and SPP signals. It isdefined as follows.Δ_(SPP)=(SPP−MPP)/S_(MPP)  (3)

where Δ_(SPP) represents the amount of SPP signal fluctuation at acertain time, and S_(MPP) represents the amplitude of the MPP signal.

Since Δ_(SPP) is a quantity which changes with time, it is necessary tomeasure a distribution of Δ_(SPP) values over a certain period of time(e.g. time taken per revolution of a disk). FIG. 6B shows a distributionof SPP signal fluctuation. From FIG. 6B, it is known that the peak ofthe distribution is not at a center of the distribution. To avoid beingaffected by SPP signal leakage resulting from interlayer crosstalk in adual-layer disk, it can be said that adjusting the offset, not to acenter of the distribution, but to an average between the maximum andminimum fluctuations as shown in FIG. 6B is effective. In the presentcase, the SPP signal fluctuation is about 60% of the MPP amplitude.

With reference to FIGS. 7A to 7C, how to measure the decentering of adisk will be described. FIG. 7A shows the MPP signal over a diskrevolution period. The amount of decentering of the disk can be obtainedbased on the track cross count of the MPP signal and the groove pitch ofthe disk using the following equation.D=P×N/2  (4)

where D represents a p-p value of decentering, P represents a trackpitch (distance between grooves), and N represents a track cross countof the MPP signal.

The track cross count can be measured by reading, after digitization,for example, by an A/D converter, the MPP signal and processing it bysoftware. In the case of a disk drive, the drive is desired toincorporate an LSI which has a measurement supporting function. FIG. 7Bis a block diagram showing an example configuration of an LSI used formeasuring the decentering of a disk. The push-pull signal (preferably anMPP signal) outputted from an optical head 110 undergoes offset and gainadjustment at an offset and gain adjustment circuit 10. At a zero-crossdetector 20, the logic pulse level changes every time a zero point iscrossed, and the pulse level edges are counted by a counter 21. Arevolution gate control circuit 22 receives a synchronization pulse froma spindle motor 160 and generates a gate signal for a disk revolutionperiod. The gate signal thus generated is used to control the measuringperiod of the counter 21. A CPU 140 calculates the amount of decenteringof the disk using the above equation and based on the count given by thecounter 21 and the groove pitch of the disk. As shown in FIG. 7C, thezero-cross detector 20 capable of track cross counting can be configuredusing a low-pass filter for noise elimination and a comparator.

FIG. 8 shows a relationship between the lens shift caused by diskdecentering and the resultant offset of the MPP signal. The relationshipis based on a simulation made by the linear diffraction method with theobjective lens diameter set to 4 mm and the numeric aperture set to0.85. The points shown beside the simulation curve in FIG. 8 representmeasurements obtained by using a BD drive. As seen from FIG. 8, theamount of an MPP signal offset which results when decentering of thedisk causes the objective lens to be moved in a radial direction of thedisk and shifted from the center of the optical beam can be known eitherbased on measurements or from a simulation.

FIG. 9 shows how a tracking control gain is adjusted. To performtracking control appropriately, it is necessary to adjust the servo gainof the generated VDPP signal. Referring to FIG. 9, MPP and SPP signalsoutputted from the optical head 110 are inputted to a tracking errorsignal generator 30, causing a VDPP signal to be generated. A trackingservo control circuit includes a gain controller 41, a phase correctionfilter 42, and a current driver 43. It performs optical spot trackingcontrol by controlling the current to flow through an objective lensactuator 114 included in the optical head 110. For gain adjustment, asinusoidal signal generated at a disturbance signal source 44 is addedto the VDPP signal, and the VDPP signal added to by the sinusoidalsignal is inputted to the gain controller 41. At the same time, thesuppression residual of the corresponding frequency is measured at theerror detector 45. This makes it possible to adjust the gain setting ofthe gain controller 41 so as to achieve an appropriate degree ofdisturbance suppression.

FIG. 10 is a diagram showing an example configuration of a trackingerror signal generator. The photo-detector 113 having eight divisionsreceive a main beam and sub-beams, and obtains an MPP signal 51 and anSPP signal 52 using a current-to-voltage conversion amplifier and adifferential amplifier configured as shown in FIG. 10. The MPP signal isinputted to an offset adjustor 31 included in the tracking error signalgenerator 30. At the offset adjustor 31, the electrical offset,generated by the current-to-voltage conversion amplifier, of the MPPsignal is corrected. Then, at the gain adjustor 33, the amplitude of theMPP signal is adjusted to a prescribed value. Similarly, the SPP signalpasses through an offset adjustor 32 and a gain adjustor 34 to undergooffset and gain adjustment. An adder 35 adds the MPP signal and the SPPsignal, and outputs a VDPP signal 53.

The mixing ratio α for the VDPP signal can be set by gain adjustment atthe gain adjustors 33 and 34. Once the gains (G°_(MPP) and G°_(SPP))that equalize the amplitudes of the MPP and SPP signals are determined,the mixing ratio α can be set to a desired value thereafter. Forexample, the mixing ratio α can be set to 0.5 by setting the gainsG°_(MPP) and G°_(SPP) at the gain adjustors 33 and 34. To set the mixingratio α to a desired value, the following equation can be used.(G_(MPP), G_(SPP))=(2αG°_(Mpp), 2(1−α)G°_(SPP))  (5)

Now, the measurements required to optimize the mixing ratio α have beenmade, that is, the amounts of (1) SPP signal fluctuation, (2) diskdecentering, and (3) servo suppression residual have been measured.These amounts are associated with three independent events. The amountof a total offset α_(tot) dependent on the three independent eventscombined is given by the following equation.α_(tot) ²=α_(SPP) ²+α_(LENS) ²+α_(SERVO) ²  (6)

where α_(SPP) represents an SPP signal fluctuation amount, α_(LENS)represents an amount of offset resulting from lens shifting, andα_(SERVO) represents an amount of servo suppression residual.

The amount of lens shifting equals the sum of the amount of diskdecentering and the amount of tracking error of the thread motor thatmoves the optical head in the disk radial direction. For the amount oftracking error of the thread motor, the corresponding design value forthe servo system can be used. In the case of the VDPP method, the amountof offset resulting from lens shifting can be uniquely determined basedon the mixing ratio α. Namely, with the offsets of the MPP and SPPsignals being identical in amount and opposite to each other in sign(positive/negative), the offset is canceled when the DPP condition(α=0.5) is met. Hence, the following equation is established.α_(LENS)=|α−0.5|×α°_(LENS)  (7)

where α°_(LENS) represents the amount of an MPP signal offset, as shownin FIG. 8, resulting from lens shifting.

FIG. 1 is a diagram showing experiment results providing a design valueof the mixing ratio α. In this case, calculations were made based on amaximum lens shift of 100 μm. As shown in FIG. 1, the offset is minimumwhen the mixing ratio α is about 0.8. The drive used in the experimenthad no previous record of successfully writing/reading informationon/from an entire dual-layer disk. It was because of tracking controlfailure during recording. After the mixing ratio α was set to 0.67 basedon the experiment results (not 0.8 because of limitation imposed bysetting specifications for front end LSI), the drive became capable ofwriting/reading information on/from an entire dual-layer disk. Thisproves the effectiveness of the present method.

FIGS. 17A and 17B show results of optical constant identificationperformed for an optical simulation using seven DVD±R/RW disks purchasedon the market. FIG. 17A shows calculated push-pull signal amplitudes.FIG. 17B shows calculated 8T signal modulation degrees. In each of FIGS.17A and 17B, the horizontal axis represents mark phase and the verticalaxis represents reflectivity ratio between mark and space.

The constants required for an optical simulation are two, i.e. recordingmark reflectivity and phase. The two constants not yet known can beidentified by measuring, in a recorded area, the push-pull modulationdegree (push-pull signal amplitude/groove level) and the data modulationdegree (data signal amplitude/groove level). To measure them, the markwidth and the groove depth are required to be known. In the presentcase, the mark width was assumed to be a 3T length, i.e. 400 nm. Thegroove depth was estimated based on the push-pull modulation degree inan unrecorded area. Furthermore, considering that a main purpose of thesimulation is to make calculations concerning the push-pull signal, thefollowing approach was used. The frequency band of the servo system ison the 1-kHz order to be equivalent to about 1/1000 of the data systemfrequency band. Therefore, for push-pull signal calculations, it isnecessary to obtain an average calculation result on recording datapatterns of 1000 Tw or more. Such a method which involves large-scalecalculations is not preferable. According to coding rules, the existenceprobability in a data signal is ½ for marks as well as for spaces. Thisis the same for DVDs and BDs. Therefore, it is good enough, for thepurpose of making calculations about the push-pull signal in a recordedarea, to determine the reflectivity and phase of a virtual data string(virtual random pattern) including 50% marks and 50% spaces in a mixedstate and apply them to the calculations. The reflectivity of such avirtual random pattern can be determined by measurement as an averagevalue of the reflectivities of the marks and spaces.

According to the results of measurement on the DVD±R/RW disks purchasedon the market, the push-pull signal amplitude was about 0.25 and the 8Tsignal modulation degree was about 0.35. These values correspond to the0.2-0.3 portion shown in FIG. 17A and the 0.3-0.45 portion shown in FIG.17B. The portion marked RW in each of FIGS. 17A and 17B is where the0.2-0.3 portion shown in FIG. 17A and the 0.3-0.45 portion shown in FIG.17B overlap. Namely, the push-pull signal amplitudes and 8T signalmodulation degrees falling in the portion marked RW agree with themeasurement results. The mark phase and reflectivity values applied tothe simulation fall in the portion marked RW. Similarly, the portionmarked R in each of FIGS. 17A and 17B corresponds to the actualmeasurements obtained from the DVD±R/RW disks purchased on the market.The mark phase and reflectivity values applied to the simulation fall inthe portion marked R.

In each of FIGS. 17A and 17B, the reflectivity is represented by theratio between the reflectivity Rm of the recorded area and thereflectivity Rs of the unrecorded area. For both DVD±R and DVD±RW media,the reflectivity of the recorded area was observed to have decreased toabout 0.5 to 0.6. As for the phase, whereas the phase of the DVD±RWmedia was approximately zero, the phase of the recorded area of DVD±Rmedia was about 0.02λ. Generally, the push-pull signal reduction causedby recording on a DVD±R disk is small. This is presumably because thephase of the recorded area is oriented in the direction to make thegroove deeper (to make the push-pull signal greater).

Using the optical constants thus obtained, stabilization effects in anOPC area were optically simulated. FIG. 11A is a diagram showing resultsof simulation performed with a variable mixing ratio α in the OPC area.It is known from FIG. 11A that changing the signal waveform only atsingular points in a border area between the recorded and unrecordedareas is possible by adjusting the mixing ratio α. This is attributableto the fact that the two sub-beams are radially shifted by half a trackpitch each from the main beam. In FIG. 11B, VA and VB each denote asingular point signal level, and ΔA and ΔB denote the differencesbetween the VA and a corresponding normal signal level and between theVB and a corresponding normal signal level, respectively. FIG. 11C showschanges in VA and VB caused by changing the mixing ratio α. When the VAand VB are equalized, the pull-in operation for reproduction isstabilized. In the present case, setting the mixing ratio α to about 0.2stabilizes the pull-in operation. FIG. 11D shows changes in ΔA and ΔBcaused by changing the mixing ratio α. When the ΔA and ΔB both approachzero, the tracking operation during recording is stabilized. Thisindicates that, in the present case, setting the mixing ratio α to about0.6 stabilizes the tracking operation.

The above simulation results relate to standard recording data collectedusing a disk drive. In cases where the recording power or the recordingfilm sensitivity varies, the recoding mark width changes to produceresults as shown by broken lines in FIGS. 11C and 11D. Particularly, inthe case of an OPC area where the recording power is varied to learn anoptimum recording power, mark width changes caused by recording powervariations become remarkable causing the mixing ratio α to change. Whenapplying the simulation results loan optical disk apparatus, it isnecessary to take into account the range of changes in the mixing ratioα resulting from changes in the recording power. Still, it remainsadvisable to keep the mixing ratio α greater than 0.5 during recordingso as to allow the optical spot to scan the track center and smallerthan 0.5 during reproduction so as to give priority to trackingstability.

Using the optical disk apparatus and tracking method according to thepresent invention improves tracking control performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing experiment results providing a design valueof a mixing ratio α.

FIGS. 2A and 2B are diagrams showing results of measuring push-pullsignals during reproduction from a dual-layer disk.

FIG. 3 is a schematic diagram showing the theory of interlayer crosstalkgeneration.

FIG. 4 is a diagram showing a DPP signal in an OPC area.

FIG. 5 gives an enumeration of causes for a tracking offset.

FIG. 6A is a diagram for explaining how to measure SPP signalfluctuation.

FIG. 6B is a diagram showing a distribution of SPP signal fluctuation.

FIG. 7A is a diagram showing an MPP signal over a disk revolutionperiod.

FIG. 7B is a block diagram showing an example configuration of an LSIused for measuring decentering of a disk.

FIG. 7C is a diagram showing a configuration of a zero-cross detector.

FIG. 8 is a diagram showing a relationship between a lens shift causedby disk decentering and an MPP signal.

FIG. 9 is a diagram showing how to adjust a tracking control gain.

FIG. 10 is a diagram showing an example configuration of a trackingerror signal generator.

FIG. 11A is a diagram showing results of DPP signal simulation performedwith a variable mixing ratio α in an OPC area.

FIG. 11B is a diagram showing singular point definition.

FIG. 11C is a diagram for explaining designing of a mixing ratio α forreproduction operation.

FIG. 11D is a diagram for explaining designing of a mixing ratio α forrecording operation.

FIG. 12 is a flowchart of processing during reproduction from adual-layer disk.

FIG. 13 is a flowchart of processing in an OPC area.

FIG. 14 is a block diagram of servo control.

FIG. 15 is a diagram showing an example configuration of a sequencer forchanging the mixing ratio α.

FIG. 16 is a block diagram showing an example configuration of anoptical disk apparatus.

FIGS. 17A and 17B show results of optical constant identificationperformed for an optical simulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in detail with reference to apreferred embodiment.

FIG. 16 is a block diagram showing an example configuration of anoptical disk apparatus according to the present invention. An opticaldisk medium 100 is rotated by a spindle motor 160. For a read operation,an electric current controlled by a laser power/pulse controller 120 isapplied to a laser diode 112 included in an optical bead 110, and alaser beam is generated. At this lime, the laser power/pulse controller120 controls the electric current such that the laser beam has a lightintensity prescribed by a CPU 140. The laser beam is split into a mainbeam and two sub-beams by a diffraction grating 116. The split beams arethen converged by an objective lens 111 to form an optical spot 101 onthe optical disk medium 100. The optical spot 101 includes a main spotformed by converging the main beam and two sub-spots formed byconverting the two sub-beams. The two sub-spots are positioned, beingshifted by half a track pitch each, on both sides of the main spot. Thelight reflected from the optical spot 101 is detected, via the objectivelens 111, by a photo-detector 113. The photo-detector 113 includes, asshown in FIG. 10, plural photo-detecting elements.

A signal processor 130, while reproducing information recorded on theoptical disk medium 100 using the signal detected by the optical head110, generates an MPP signal and an SPP signal, from which it thengenerates a VDPP signal. A servo-controller 150 performs focusing andtracking. For tracking, the servo-controller 150 uses the VDPP signal.Focusing is performed based on a focus error signal generated from amain spot detection signal. A reproduction signal is generated from themain spot detection signal. For recording, the laser power/pulsecontroller 120 converts prescribed recording data into a prescribedrecording pulse current, and performs control so that pulsed light isemitted from the laser diode 112. The circuit required for using thetracking method according to the present invention is incorporated inthe signal processor 130. Processing for learning the mixing ratio αreferred to in the foregoing is executed as a program stored in the CPU140. The optical disk apparatus of the present invention can be providedbased on the configuration described above.

First, the tracking method according to the present invention will bedescribed. FIG. 12 is a flowchart of the tracking method according tothe present invention used in reading a dual-layer disk. The initialvalue of the mixing ratio α is 0.5. The MPP and SPP signals are measuredin a focused state, and the fluctuation of the SPP signal is calculated.Shifting of the lens is then measured. When the SPP signal fluctuationis smaller than a threshold value, the mixing ratio α is left unchangedat 0.5, and tracking servo control is performed. When the SPP signalfluctuation is larger than the threshold value, an optimum value of themixing ratio α is determined based on the amount of the offset due todisk decentering and the amount of the fluctuation, and the residualtracking offset is estimated. When the residual tracking offset becomessmaller than a threshold value, the tracking servo control is performedusing the mixing ratio α thus determined. When the residual trackingoffset is larger than the threshold value, the condition is regarded asan error.

FIG. 13 is a flowchart of the tracking method according to the presentinvention performed in a single-layer OPC area. The process up to thestep where the SPP signal fluctuation is calculated is the same as shownin FIG. 12. Subsequently, whether or not the current operation isperformed in an OPC area is determined. When the current operation isbeing performed in other than an OPC area, the mixing ratio α is set to0.5. When the current operation is being performed in an OPC area, themixing ratio α is set to a value lower than 0.5, for example, 0.2 forreproduction or a value higher than 0.5, for example, 0.7 for recording.For the mixing ratio α, plural appropriate values may be stored inadvance so that one of them may be selected for use according to thecondition of operation.

FIG. 14 is a block diagram of preferred servo control for the opticaldisk apparatus according to the present invention. Referring to FIG. 14,control of the objective lens actuator by the VDPP signal is asdescribed in the foregoing. FIG. 14 also shows a servo control loop fora thread motor. The objective lens is driven by the objective lensactuator and the optical head is driven by the thread motor both in adirection transversal to the track, namely, in a radial direction of theoptical disk. If the thread motor is controlled so that the offset ofthe VDPP signal approaches zero, the signal offset relative to the lensshift will become smaller than appropriate, for example, in a case wherethe mixing ratio α is set to 0.5. Controlling the thread motor in such away is not appropriate. To realize stable position control for thethread motor without depending on the value of the mixing ratio α, it isappropriate to control the thread motor so that the offset of the MPPsignal approaches zero. Namely, the thread servo control is to beperformed using the MPP signal that is sensitive (showing a largeoffset) to disk decentering.

FIG. 15 is a diagram showing an example configuration of a sequencer forchanging the mixing ratio α in the optical disk apparatus according tothe present invention. According to the present invention, it isnecessary to change the mixing ratio α between when accessing an OPCarea and when recording data. Instantly changing the mixing ratio αmakes tracking control unstable. Referring to FIG. 15, gain settingsstored in a data array 142 by a sequencer 141 are automaticallytransferred to a tracking error signal generator 30 as serial data 54 atappropriate intervals. Changing the mixing ratio α in steps as in thiscase secures tracking control stability.

The present invention is applied to an optical disk apparatus in whichtracking control is performed using three beams.

1. An optical disk apparatus, comprising: an optical head including alight source, a beam splitter which splits a beam from the light sourceinto a plurality of beams, an objective lens which converges theplurality of beams split by the beam splitter into a plurality ofoptical spots on an optical disk, a photo-detector which detects theplurality of optical spots reflected from the optical disk, and anactuator which drives the objective lens in a radial direction of theoptical disk; a thread motor which drives the optical head in the radialdirection of the optical disk; a unit which generates a plurality oftracking spot signals from detection signals generated by detecting theplurality of optical spots; a unit which generates a tracking controlsignal for driving the actuator by mixing the plurality of tracking spotsignals; and a mixing ratio changing unit which changes a mixing ratioof the plurality of tracking spot signals; wherein the mixing ratiochanging unit determines the mixing ratio based on a measured amount offluctuation of each of the tracking spot signals and a measured amountof decentering of the optical disk.
 2. The optical disk apparatusaccording to claim 1; wherein a plurality of values of the mixing ratioare stored in advance, and the mixing ratio changing unit uses a valueselected from the plurality of values stored in advance.
 3. The opticaldisk apparatus according to claim 1; wherein the mixing ratio changingunit changes the mixing ratio in steps.
 4. The optical disk apparatusaccording to claim 1; wherein the plurality of optical spots include amain spot and two sub-spots positioned, being shifted by half a trackpitch each, on both sides of the main spot, and the thread motor iscontrolled using a tracking spot signal which is generated by adetection signal generated by detecting the main spot.
 5. The opticaldisk apparatus according to claim 1: wherein the plurality of opticalspots include a main spot and two sub-spots positioned, being shifted byhalf a track pitch each, on both sides of the main spot; wherein theunit that generates a plurality of tracking spot signals generates amain push-pull signal from a detection signal generated by detecting themain spot and a sub-push-pull signal from detection signals generated bydetecting the sub-spots; and wherein the mixing ratio changing unitchanges the mixing ratio based on an initial state where gain correctionhas been made to equalize the main push-pull signal and thesub-push-pull signal in amplitude at a time of crossing a track of theoptical disk.
 6. A tracking method of an optical disk apparatus whichirradiates an optical disk with a plurality of optical spots andgenerates a tracking signal from detection signals generated bydetecting light reflected from the plurality of optical spots, thetracking method comprising the steps of: generating a tracking signalwith a variable mixing ratio by adjusting outputs of a plurality oftracking error signals, which are generated from detection signalsgenerated by detecting the light reflected from the plurality of opticalspots, and by performing addition or subtraction using the plurality ofadjusted tracking error signals; changing the mixing ratio according toan operating condition of the optical disk apparatus; and controllingpositions in a disk radial direction of the plurality of optical spotsby using the tracking signal with a variable mixing ratio; wherein theplurality of optical spots include a main spot and two sub-spotspositioned, being shifted by half a track pitch each, on both sides ofthe main spot; and wherein the step of changing the mixing ratioaccording to an operating condition of the optical disk apparatusincludes obtaining an amount of fluctuation of a second tracking errorsignal which is generated from detection signals generated by detectingthe two sub-spots and an amount of shifting of an objective lens whichconverges the plurality of optical spots on the optical disk, obtaining,when the amount of fluctuation is greater than a predetermined thresholdvalue, an amount of offset of the second tracking error signal resultingfrom the shifting of the objective lens, and obtaining the mixing ratiobased on the amount of offset and the amount of fluctuation.
 7. Thetracking method according to claim 6: wherein a plurality of values ofthe mixing ratio are stored in advance, and a value selected, accordingto an operating condition of the optical disk apparatus, from theplurality of values stored in advance is used.
 8. The tracking methodaccording to claim 6: wherein the plurality of optical spots include amain spot and two sub-spots positioned, being shifted by half a trackpitch each, on both sides of the main spot; and wherein the mixing ratiois set such that, when the optical spots are engaged in reproductionoperation in an OPC area, a ratio of a tracking error signal which isgenerated from detection signals generated by detecting the twosub-spots increases, and such that, when the optical spots are engagedin recording operation in the OPC area, s ratio of a tracking errorsignal which is generated from a detection signal generated by detectingthe main spot increases.
 9. A tracking method of an optical diskapparatus which irradiates an optical disk with a plurality of opticalspots and generates a tracking signal from detection signals generatedby detecting light reflected from the plurality of optical spots, thetracking method comprising the steps of: generating a tracking signalwith a variable mixing ratio by adjusting outputs of a plurality oftracking error signals, which are generated from detection signalsgenerated by detecting the light reflected from the plurality of opticalspots, and by performing addition or subtraction using the plurality ofadjusted tracking error signals; changing the mixing ratio according toan operating condition of the optical disk apparatus; and controllingpositions in a disk radial direction of the plurality of optical spotsby using the tracking signal with a variable mixing ratio; wherein themixing ratio is changed in steps.
 10. The tracking method according toclaim 6: wherein the plurality of optical spots include a main spot andtwo sub-spots positioned, being shifted by half a track pitch each, onboth sides of the main spot; and wherein the step of generating atracking signal with a variable mixing ratio includes generating a mainpush-pull signal from a detection signal generated by detecting lightreflected from the main spot and a sub-push-pull signal from detectionsignals generated by detecting light reflected from the sub-spots; andperforming gain adjustment to generate an initial state where the mainpush-pull signal and the sub-push-pull signal have an equal amplitude ata time of crossing a track of the optical disk.